I have a very long (30m), narrow garden. The soil is very poor, sandy and stony. I have been battling bindweed for 40 years. I have tried poisoning it, digging it up, smothering it with black plastic, but the minute I turn my back up it comes! I should like to turn half the garden into a wild flower meadow, but fear the bindweed will come up through any wild flowers I grow. How can I get rid of it?